This invention relates to a process for electrolytically converting sulfur compounds dissolved in aqueous liquids to elemental sulfur. The invention further relates to a process for removing SO.sub.x compounds from stack gases by absorption in an aqueous absorbent and electrochemically regenerating the resulting spent absorbent.
It is known in the art of sulfur chemistry that SO.sub.x compounds (i.e., SO.sub.2 plus SO.sub.3) can be removed from stack gas streams by absorption in alkaline, aqueous liquids, e.g., an aqueous solution of sodium hydroxide. It is further known that spent absorbent solutions obtained from such absorption processes, containing, for example, large concentrations of sodium cations and assorted sulfur-containing anions, may be electrochemically regenerated so as to produce a fresh absorbent solution of sodium hydroxide. Processes typifying this approach are shown in U.S. Pat. Nos. 3,607,001 and 3,515,513. In these processes, the spent absorbent solution is first treated, as by stripping or heating, to desorb as much SO.sub.2 as possible, thereby producing a concentrated stream of SO.sub.2 suitable as one component of a Claus plant feed. The remainder of the spent absorbent, largely comprising an aqueous solution of sodium sulfate, is directed to an electrolytic cell containing two diaphragms (or ion-permeable membranes) separating a feed compartment from anode and cathode compartments. Under the influence of an impressed direct electric current, the sulfate anions migrate from the feed compartment through one diaphragm to the anode compartment, producing therein an anolyte solution comprising sulfuric acid. Simultaneously, cations pass through another diaphragm to the cathode compartment, producing a catholyte solution of sodium hydroxide. Hence, in the usual case, the products removed from the electrolytic cell comprise a sulfuric acid solution and a caustic or other alkali metal hydroxide solution useful as fresh absorbent for removing SO.sub.2 from the stack gas.
Several problems are involved in using the electrochemical processes as above described. First, considerable power loss occurs across the diaphragms, thereby reducing the efficiency of the cell. Further, the production of sulfuric acid from such processes is usually undesirable because sulfuric acid is not an economic product to store or transport when produced in large quantities. Moreover, because that portion of the spent absorbent fed to the electrolytic cell contains sulfite, bisulfite, and bisulfate ions as well as sulfate ions, the sulfuric acid produced from the electrolytic cell is impure, and thus of much less economic value than more purified forms of sulfuric acid.
In view of the foregoing, it would be far more desirable to electrochemically convert the sulfate and other oxysulfur anions in such spent absorbent solutions to a single product, preferably a solid product such as elemental sulfur, which is more easily stored and more marketable than sulfuric acid. However, producing sulfur in a typical diaphragm cell raises the obvious problem that the elemental sulfur will easily plug the ion membrane pores and thus render the cell inoperative. Additionally, sulfur could collect around the cathode of the cell and thus interfere with the efficiency of the cell for the intended conversion.
Accordingly, it is an object of the invention to electrochemically produce elemental sulfur in a diaphragmless cell in which one or more sulfur-containing anions are converted to the single, homogeneous product of elemental sulfur. It is a further object to provide a process wherein, by electrochemical conversion in a diaphragmless, mercury, electrolytic cell, the sulfur-containing anions in spent aqueous absorbents recovered from processes for removing SO.sub.2 and SO.sub.3 from stack gases are converted to elemental sulfur. It is yet another object to provide a diaphragmless, mercury, electrolytic cell in which elemental sulfur is produced from an aqueous electrolyte without the elemental sulfur collecting around the electrodes, especially the mercury electrode, and thus interfering with the cell efficiency. Other objects and advantages inhering in the invention will become apparent to those skilled in the art from the following detailed description.